1. Field of the Invention
This invention relates to systems and methods for the control of spacecraft such as satellites. Particularly, this invention relates to systems and methods for pointing control of satellites.
2. Description of the Related Art
Precision attitude control of spacecraft is critical to provide accurate pointing of various antennas, sensors and other payloads. Imaging spacecraft such as intelligence satellites and weather satellites require precision payload pointing to achieve their mission objectives. Communication satellites require precision pointing of their RF and optical crosslinks to establish communication links between satellites. In addition, space based weapon platforms require precise payload pointing to target accurately. These various payloads are usually gimbaled and are precisely pointed by their gimbals control systems.
However, various influences introduce disturbances (e.g. torque disturbances) into such control systems. For example, disturbances can arise from harness across moving joints (harness residual torque and stiffness) and also from gimbal and motor ripples and cogging. Related, but distinct are the disturbances from gimbal, motor and harness running friction. In addition, there are also disturbances due to gimbal, motor and harness friction hysteresis. Thus, some major sources of pointing error for gimbaled payloads are the result of control disturbances within the gimbals themselves. They usually reduce the effectiveness of a gimbals control system, and thus may significantly decrease pointing accuracy of a gimbaled payload. The control system and spacecraft design must compensate for the negative effects of these influences.
Typically in the prior art, to reduce disturbances very high quality gimbals components and designing high bandwidth control systems are employed. Such high quality gimbals components are very expensive. In addition, though they generate fewer disturbances, these disturbances are still limiting factors of the pointing performance of a gimbals control system. Furthermore, high bandwidth spacecraft control systems are capable of achieving higher pointing accuracy. However, the improved pointing accuracy is usually achieved at the expense of control system robustness to various uncertainties of the control system. In addition, the bandwidth of a gimbaled control system is often limited by phase delays inherent in such a control system.
In view of the foregoing, there is a need in the art for high bandwidth control systems and methods which improve pointing performance. Further, there is a need for such control systems and methods to operate robustly. There is also a need for such control systems and methods to reduce the impact of phase delays in order to improve performance. As detailed hereafter, these and other needs are met by the present invention.